The Exposure Assessment and Modeling Core (EAMC) will provide technical support to Center projects through the collection, analysis, and development of models describing intra-community variability of air pollutants with potential health impacts. Based on questionnaire responses from subjects recruited in a larger cohort study in 13 southern California communities, asthmatic subjects (n approximately 160) and matched controls (n approximately 160) will be invited to participate in an ambient air pollution sampling study at their respective residences. Measurements of ambient ozone, nitric oxide, and nitrogen dioxide will be recorded for five two-week sampling periods outside each subject's home, with concurrent monitoring at neighborhood schools and the central community monitoring station. These field data will be utilized to develop spatially refined and regionally applicable models of pollution levels for use in relating health outcomes to exposure indices. Traffic density models and geo-coded sampling locations will be exploited to enhance the modeling approach and provide intra-community specific exposure estimates for subject assignment.